Titania particles possess an attractive combination of optical properties such as absorption of ultraviolet light and a very high refractive index. Polymer composites comprising inorganic nanoparticles are attractive for a range of optical applications (Carotenuto et al., 1996). Undesired light scattering (Beecroft and Ober, 1997) is significantly reduced in nanocomposites compared to composites containing larger particles (>50 nm; Kyprianidou-Leodidou et al., 1997) if the refractive indices of polymer and particles differ and if the primary particles are randomly distributed in the polymer matrix. These nanocomposites will often appear transparent (Caseri, 2006). Rutile TiO2-based nanocomposites can be used as UV filters, coatings for UV-sensitive materials and lenses as the particles absorb UV light, are transparent at the visible wavelengths and possess a high refractive index (Nussbaumer et al., 2003). The anatase phase is less suited for these applications as its absorption edge is located at lower wavelengths (Christensen et al., 2000) and generally has a higher photocatalytic activity which can lead to degradation of the polymer matrix (Allen et al., 1992).
A significant amount of the world's annual production of titania is produced by the gas phase oxidation of titanium tetrachloride using a pyrolysis process. In flame synthesis of titania, an industrial process for the production of pigmentary titania (Braun, 1997), typically the anatase phase is formed under oxygen-rich conditions at atmospheric pressure (Wegner and Pratsinis, 2003). Rutile can be synthesized by thermal treatment of anatase titania (Song and Pratsinis, 2000), however this also leads to grain growth and agglomeration. The larger particles or agglomerates significantly scatter light (Beecroft and Ober, 1997) resulting in opaque composites (Nussbaumer et al., 2003; Caseri, 2006). Rutile formation during synthesis of titania nanoparticles can be promoted by co-oxidation with aluminum precursors (Mezey et al., 1966) as has been shown in hot-wall (Akhtar et al. 1994) and diffusion flame (Vemury and Pratsinis 1995) aerosol reactors.
The titania particle surface can be passivated by coatings, in order to prevent the phototcatalytic decomposition of the polymer matrix (El-Toni et al., 2006). Coatings can reduce the generation of free radicals by physically inhibiting oxygen diffusion, preventing the release of free radicals and providing hole-electron or hydroxyl-radical recombination sites (Allen et al., 2005). Furthermore, coatings can also improve wetting and dispersion properties of the particles in an organic matrix (Egerton, 1998; Allen et al., 2005). Coatings are typically applied to pigmentary titania in a post-synthesis, wet-phase treatment by precipitation of nano-sized hydrous oxides of Al, Zr, Sn or Si onto the titania surface (Iler, 1959). Silica coating of titania is particularly attractive because this coating yields maximum durability of the coated material. However, this is also accompanied by loss of opacity as a result of agglomeration during wet-phase treatment. Wet dispersion of the starting powder, filtration, washing and drying add to production time and cost. Furthermore, the control of the coating morphology is difficult in the wet precipitation process. Rough and porous coatings are often obtained where complete and homogeneous coatings are desired for optimum durability and a maximum reduction of photoactivity of the titania.
In-situ gas-phase processes have been investigated as alternative coating routes either in aerosol flow (Piccolo et al., 1977) or flame reactors (Hung and Katz, 1992). In flame reactors SiO2 coated TiO2 can be formed by co-oxidation of silicon and titanium precursors (Hung and Katz, 1992; Teleki et al., 2005). The product powder morphology is a result of simultaneous growth of the two oxides in the flame and can be controlled by precursor concentration and flame temperature (Hung and Katz, 1992). In a diffusion flame rapid cooling of particle growth by nozzle quenching (Wegner and Pratsinis, 2003) facilitated the formation of smooth silica coatings while in the unquenched flame mainly particles segregated in silica and titania were formed (Teleki et al., 2005). In aerosol flow reactors coating precursors can be added downstream a TiO2 particle formation zone to produce oxide coatings on the titania nanoparticles (Kodas et al., 1996; Powell et al., 1997). The key process parameters controlling coating morphology are temperature and coating precursor concentration (Powell et al., 1997) as well as the mixing mode of titania particles and coating precursor (Lee et al., 2002).
U.S. Pat. No. 5,562,764 to Gonzalez describes a process for producing substantially anatase-free TiO2 by addition of a silicon halide to the reaction product of TiCl4 and an oxygen containing gas in a plug flow reactor. The silicon halide is added downstream of where the TiCl4 and oxygen gas are reacted. The patent describes a process to produce pigmentary grade TiO2, and the SiCl4 is added to the process at a temperature of about 1200° C. to about 1600° C., and a pressure of 5-100 psig.
International Application Publication No. WO 96/36441 to Kemira Pigments, Inc. describes a process for making pigment grade TiO2 coated with a metal oxide in a tubular flow reactor. The metal oxide precursor is introduced downstream of the TiO2 formation zone. The publication discloses that the temperature for treating TiO2 with a silica precursor must be sufficiently high to ensure that the precursor forms SiO2. The publication discloses that for coating TiO2 with SiO2 using SiCl4, the temperature must be greater than 1300° C. The particles produced by the process are pigment grade.
U.S. Pat. No. 6,562,314 to Akhtar et al., describes a process for the production of substantially anatase-free TiO2 by introducing a silicon compound into the TiCl4 stream to form an admixture before the reaction with oxygen. The process is conducted under pressure and the titania is not coated with silica.
U.S. Pat. Nos. 6,852,306 and 7,029,648 to Subramanian et al., describe a process to produce TiO2 pigment particles coated with silica in a tubular flow reactor. The TiCl4 is introduced downstream of the TiO2 formation zone at a temperature of no greater than 1200° C. The coating produced by this about 1 to 4 nm thick and is a mixture of amorphous aluminum oxide and amorphous silicon dioxide. Only silicon halides are used as the metal oxide precursor.
U.S. Pat. No. 7,083,769 to Moerters et al., describes silicon-titanium mixed oxide powders prepared by a flame hydrolysis process. The process described comprises introducing streams of TiCl4 and a silica precursor into the burner at the same time. The mixed oxide produced is disclosed to be an intimate mixture of titanium dioxide and silicon dioxide on an atomic level with the formation of Si—O—Ti bonds. The surface of the particles is disclosed to be enriched with silicon.
U.S. Pat. No. 6,328,944 to Mangold et al., describes doped metal oxides or non-metal oxides prepared by a process which comprises feeding aerosols into the flame of a pyrogenic reactor. SiCl4 is fed into a combustion chamber via one feed tube and an aerosol which comprises a second metal oxide dopant is fed to the combustion chamber separately through another tube. The SiCl4 and the dopant aerosol are mixed together prior to reaching the combustion chamber.
Although these gas-phase coating techniques offer promise in obtaining metal oxide coated titania particles with desired characteristics, the production of titania nanoparticles with smooth homogeneous metal oxide coatings remains a challenge. Therefore, there remains a need for a pyrolysis process that produces high rutile content titanium dioxide nanoparticles that are coated with a smooth and homogeneous coating of a second metal oxide layer.